Grounding for shed with solar

[ShaneC]

Hi all,

I am building a shed in the back garden in a couple of weeks, I have purchased 6x415W solar panels for the roof. They will be arranged in a 3S2P configuration. I will be creating a combiner box with fuses, breaker and SPD. In the shed will be the SCC and Inverter, both requiring grounding. On the DC side then there are the panels themselves, SPD, SCC and Inverter that will need to be grounded.

I plan on running backup power from the house that will go through a 2P ATS. My question is can I ground the DC side components to a grounding rod buried by the shed but ground the AC side using the house's earth cable? Given that the ATS I have is only 2P it won't switch earth. Ideally I would just bond earth from the house to the neutral coming from the inverter in the consumer unit, then just the live/neutral would be switched, either coming from the inverter or from the house.

Would bonding earth to another neutral potentially trip the RCD in the main house consumer unit?

Could having two earths, one on the DC side and one on the AC side still leave the potential for a lightning shock to pass through from one to the other?

Alternatively I could ground everything on the shed side to the earth rod, AC included and buy a 3P ATS so that the house circuit never comes into contact with the shed circuit.

Many thanks for any advice given, I've figured out most things through research and searching the forums here but as always grounding seems to be the complex subject.

 

[FilterGuy]

I plan on running backup power from the house that will go through a 2P ATS. My question is can I ground the DC side components to a grounding rod buried by the shed but ground the AC side using the house's earth cable?

That would not be to code.

All grounding conductors should go back to the grounding at the house.

The question becomes whether there should be an additional ground rod at the shed.

If you were running a line out to the shed and fed it into a breaker box, An additional ground rod would be required. (I don't like having separated ground rods like that, but that is the code)

Your case is slightly different in that the power is going from the shed to the house, However, I would guess that there still needs to be a ground rod at the shed.

 

[ShaneC]

Yea that was my other possibility, just using the earth from the house. I don't like the idea of a lightning strike hitting the panels and having to pass through a lot of earth cable, buried cable and then back through the house circuit and to the mains ground connection.

Can I have an earth rod solely for grounding the solar panels and SPD in the combiner box and then everything else run off the earth from the house mains?

 

[zanydroid]

My understanding is that earthing the canonical way (to house ground) is the safest way to get the necessary non-lightning protection properties. And if you want to try to modify that for lightning protection you probably need to engineer it from scratch. With the number of people that ask this exact question and the lack of a definitive answer/standard implementation, I doubt it’s easy or cheap. Granted this is not a primarily electrical/power engineer forum

Not clear either whether everyone asking about this is on underground service, thus the solar panels are a significant increase in lightning risk compared to before adding them.

To throw another random dart at it, maybe (in US terms) if you clearly make the two systems separately derived by putting an appropriate isolation transformer between them, then you can N-G bond on both sides, to separate ground rods, with no consequence. And the transformer will provide some isolation (but some energy will still conduct through due to the impulse, and the energy/voltage may well bust through the isolation rating).

I doubt there are many code authorities that would allow ground to be switched in a simple matter, it would probably have rules like high impedance grounding does.

 

[ShaneC]

Ok, so just to be clear the best way to do this safely will be to use a grounding rod purely for the lightning protection for the solar panel frames and the SPD. Everything other than that should be grounded to the mains earth?

 

[zanydroid]

Keep in mind there’s 3-5 different concepts when people talk about grounding, probably good to stick with standard practices from the code book until you can get through the painful slog of understanding enough of them.

Grounding rod and ground system are not for lightning protection.

I’m not sure the SPD can do anything if it clamps energy to just a ground rod. That ground rod could well get pulled up to a substantial % of the surge voltage. In the U.S. when a SPD is connected to neutral or ground (which is bonded on site through low impedance bond) the surge has something that is “strongly” at 0V. If the impedance through the “ground” you picked for SPD is too high relative to neutral, how can the clamping be as effective?

I believe a ground rod is only strong enough to dissipate static, and in some applications where it’s important to bring up the potential of the earth (hot tub?) to local neutral potential, the higher impedance is actually good. Not sure about this, it’s something that came to mind while watching a John Ward video couple months back.

 

[zanydroid]

Yes there could be increased lightning vulnerability but the code writers for sure considered the tradeoffs.

 

[AntronX]

I plan on running backup power from the house that will go through a 2P ATS. My question is can I ground the DC side components to a grounding rod buried by the shed but ground the AC side using the house's earth cable?

No, induced earth potential from nearby lightning strike will induce high voltage between DC and AC side of your inverter possibly damaging it. All ground rods in your system have to be bonded together via #6 bare copper wire (except those ground rods used purely for air terminal lightning rods that have nothing else connected to them).

Given that the ATS I have is only 2P it won't switch earth. Ideally I would just bond earth from the house to the neutral coming from the inverter in the consumer unit, then just the live/neutral would be switched, either coming from the inverter or from the house.

Bond inverter neutral to earth when running the shed on inverter. Unbond when switching to backup feed from house. Avoid a condition where you have 2 simultaneous neutral to earth bonds.

Would bonding earth to another neutral potentially trip the RCD in the main house consumer unit?

Not sure what you mean by main house consumer unit.

Alternatively I could ground everything on the shed side to the earth rod, AC included and buy a 3P ATS so that the house circuit never comes into contact with the shed circuit.

That's unnecessary and would offer no protection since ground potential rise from nearby strike could induce over 100 kV inside your transfer switch and arc over. Just keep 2 ground rods bonded together and this problem goes away.

Ok, so just to be clear the best way to do this safely will be to use a grounding rod purely for the lightning protection for the solar panel frames and the SPD. Everything other than that should be grounded to the mains earth?

No because now you would have potential rise between solar frame and solar cells which would not withstand 100 kV potential rise and would arc over destroying your solar panel. MPPT and inverter would in theory be protected by surge arresters but depending on severity of the lightning strike they may fail as well. Bonding everything electrical to common low impedance earth bond reduces the chances of damage.

 

[FilterGuy]

Yea that was my other possibility, just using the earth from the house. I don't like the idea of a lightning strike hitting the panels and having to pass through a lot of earth cable, buried cable and then back through the house circuit and to the mains ground connection.

Can I have an earth rod solely for grounding the solar panels and SPD in the combiner box and then everything else run off the earth from the house mains?

To be code compliant, all of the groundings must tie together and go back to the house. The only time that would not be true is if the systems were totally separate and you were not feeding power back through the ATS. (No wires between the shed and the house).

Where the code gets wonky is grounding electrodes at the shed.

1. If the wire from the house goes to a breaker box (It is a feeder circuit), There must be a ground rod at the shed.
2. If the wire from the house just goes to some lights and/or sockets but not a breaker box (A branch circuit), a ground rod at the shed is permitted but not required.

If I am understanding your description you will have a breaker box at the shed so you will need to install a grounding electrode at the shed.

Note: I don't think the code directly addresses a branch circuit in a 2ndary structure that has a power source on it. However, I suspect that by the time all the other code is met for the inverter, there will be a breaker at the shed and the wire to the shed would be considered a feeder circuit. Consequently, a grounding electrode would have to be installed at the shed.

----------------------------------------------------

A direct lightning strike will blow things up and no grounding scheme in the world will change that. However, a properly installed grounding system can help reduce the possibility of damage due to a nearby lightning strike. ( I question the wisdom of the code requiring an additional grounding electrode at the 2ndary structure..... but that is a bit of a rat-hole and the code should be followed regardless of what I think)

There are lightning protection systems, but they are completely independent of the electrical grounding system. A Lightning protection system typically involves towers and lightning rods that are grounded to the earth separately from the electrical system. In addition, they will often include aerial wires between the towers to further ensure an exclusion zone under the towers. The objective of the lightning protection system is for it to be the path for the lightning so that whatever it is protecting will not get hit.

 

[zanydroid]

Not sure what you mean by main house consumer unit.

Overseas equivalent of our panel boards.

Just keep 2 ground rods bonded together and this problem goes away.

I think what people want is to be able to air gap the two systems. Any equipment installed to provide the isolation would need to be rated to take a hit of the size someone intends to absorb, which may be a tall order.

If the hit is something that an SPD for typical residential etc can absorb, then you only need an SPD or two, and everything can be connected like the book says you should.

I am not sure how much the components for DIY solution that can take a bigger hit than the biggest SPD are going to cost…

 

[timselectric]

One grounding system for everything.
All grounding should be connected back to the (house) original grounding system.

Auxiliary ground rod at the shed?
Could be required by your local AHJ (Authority Having Jurisdiction). If not reqired or not getting inspection. I would recommend against it.
But if an auxiliary ground rod is installed. It must also be connected to the existing grounding system.

This is the electrical grounding system. Not lighting protection. Lighting protection is a completely different, and very expensive system.

 

[ShaneC]

I would say the possibility of a lightning strike is quite low, there are taller houses and trees nearby that would take the hit first so protecting from a nearby strike would be enough. Other than that I just want to make sure everything is grounded correctly. It's worth saying I am in the UK so regulations may differ.

The only reason I raised the lightning protection is due to the use of an SPD in the combiner box, I was under the impression that they were there for handling large static loads from the panels, ie a nearby lightning strike. If that makes no sense then ignore me, I'm far more interested in just making sure everything is grounded correctly.

If using the house ground would be best then I can do that, I just need to figure out a good route for the grounding cable to get from the front door where the CU (breaker panel) is to the back garden. I'll have to do that anyway to run a new circuit out there.

I was under the impression that having two separate grounding points was bad, vaguely remember it coming up in one of Will's videos. This won't be getting inspected, I plan on removing atleast the power generation (solar/inverter etc) from the shed and re-wire it just with the electrics from the house when we come to sell this place.

 

[timselectric]

SPD's are good insurance.
I recommend them for everything.

 

[ShaneC]

I can't believe I forgot about your amazing guides that I found quite a while ago @FilterGuy

Grounding Made simpler - Part 1: AC/houshold grounding

To get the paper, click on the orange button at the top of the screen. The subject of grounding is a complex, multifaceted subject, that is often treated as an after-thought but needs to be considered from the beginning of the design and build...

diysolarforum.com

Just re-reading them now and they might well answer my questions. I also found this information which relates to UK regulations:

Installing an electrical supply to an outbuilding

This article looks at the correct procedure for installing an electrical supply to an outbuilding. Before carrying out any new or additional installation works a contractor must determine that the supply to the installation is sufficient to meet the demands of the additional load. It must also...

electricalapprentice.co.uk

It seems to indicate that I can just use the mains earth for this as well, it's not specific to the issue of the solar generation but the closest I can find for now.

 

[zanydroid]

In the US the grounding (EGC) path needs to follow the feeder going to the shed, and this is done (for split-phase residential) with 4-wire for L-L-N-G. I'm pretty sure I've seen direct-bury cable YT videos from the UK for this use case including installation videos, you can see what type they use, there's some interesting armored cable that looks easy enough to work with (my guess would be you would end up with armored burial rated L-N-G). And check John Ward's videos I linked above.

If the cable needs to go under a path where heavy vehicles will need to go over, there are extra anti-crush considerations which should be covered in the code.

Are you not able to get plan check from your local council or AHJ?

 

[ShaneC]

Are you not able to get plan check from your local council or AHJ?

I'm not sure what the ahj is, the council here doesn't provide that service, I'd have to pay £100+ for an electrician to come out and take a look, more to actually do anything. I might pay one to come and check it over once I have everything installed and recommend any changes.

 

[zanydroid]

I'm not sure what the ahj is, the council here doesn't provide that service, I'd have to pay £100+ for an electrician to come out and take a look, more to actually do anything. I might pay one to come and check it over once I have everything installed and recommend any changes.

AHJ = what our code calls the local department that checks and approved permits. For wiring plans I submit the blueprints of where the conduits and cables go, and what wires go in in those conduit + their size. They then usually send this to an outside contractor to check. No clue how thorough they are with math and code compliance or whether this indemnifies against the inspector later seeing something done exactly to plans but the plan mistakes were not caught.

100 GBP sounds pretty damn good consulting fee value for electrician to tell you how to do it to code, if the information is conveyed accurately and completely.

How do you get a permitted installation over there?

 

[ShaneC]

100 GBP sounds pretty damn good consulting fee value for electrician to tell you how to do it to code, if the information is conveyed accurately and completely.

How do you get a permitted installation over there?

Ah that's £100 just to get him out, anything beyond a quick once over would be extra. Consulting from the start with certification at the end even if I did all the work would come to much more.

For anything to be officially inspected it has to be checked by a licensed electrician, they can sign it off. The council only gets involved with building work.

 

[zanydroid]

FWIW in the US I know of two forums that can give me exactly the right answer to the question you are asking, for US code, along with tips on how to optimize the cost and installation effort. Part of that comes from being a regular on those forums and knowing who to listen to, because they're right 99% of the time. Unfortunately I don't think those forums have UK electricians (and that would be a bad idea, you want a focused forum on code stuff to avoid confusion since it's very different across countries. On those forums things get confusing sometimes because both Canada and US are on there and things are both similar and different :laugh. And the quality of help is far superior from what I expect locally from 100 GBP 20 min consultation from an electrician.

Hopefully there's a forum for UK code.

 

[ShaneC]

Yea it seems that the UK code is saying definitely do not have a grounding rod in the shed that could be connected to the earth from the house. You can have a grounding rod but then you have to make sure not to connect the two earths but you can still use live and neutral from the house mains.